This invention is directed to a rotor construction for an electronic wristwatch step motor, and in particular to a rotor construction including a high performance brittle permanent magnet core, and method of forming a rotor construction including same.
Electronic wristwatches incorporating quartz crystal vibrators as a time standard have yielded wristwatches having accuracies heretofore unobtainable in mechanical movement wristwatches. Such wristwatches include a quartz crystal ocillator circuit for producing an oscillating signal in a response to the vibration of the quartz crystal vibrator. A divider circuit including a plurality of divider stages in response to the high frequency oscillating signal produced by the oscillator circuit produces a very accurate low frequency drive signal. A step motor includes a rotor having the torque thereof determined by the timing drive signals produced by the divider circuit and applied to the step motor. The rotor is rotationally coupled to a gear train assembly which assembly is mechanically coupled to the hands for displaying time. Accordingly, the rotational motion of the rotor is transmitted to the geared hands to effect time display.
One requirement of the step motor, and in particular the rotor construction thereof is that the size be limited in order to make same particularly suitable for use in a small sized wristwatch. Nevertheless, in order to effect the accurate translation of the rotational motion to the hands in response to the extremely accurate electronic signals supplied to the step motor, the rotor assembly must be formed of a high performance magnet and have good mechanical strength.
Heretofore, the use of permanent magnets such as Pt-Co magnets have been utilized in the rotor constructions of wristwatch step motors. Although such permanent magnets were expensive to manufacture, their popularity has been based on the facility with which same can be machined and the relatively good performance thereof.
Nevertheless, high performance permanent magnets such as samarium-cobalt or barium ferrite which are capable of being fabricated at minimum cost have been utilized in permanent magnet rotors of the type discussed above, because same are extremely brittle and therefore cannot be machined in the same manner as the considerably lower performance magnets discussed above. For example, the use of such high performance brittle permanent magnet materials in a rotor construction have required cutting by use of a grindstone, abrasion processing, and the caulking of the rotor construction to the rotor pinion in order to manufacture a rotor construction having a very high performance permanent magnet. The cutting and abrasion processes require considerable time to obtain the necessary accuracy. Moreover, the caulking of the permanent magnet to the rotor pinion shaft has proved less than completely satisfactory in view of the eccentricity caused by the gap between the shaft and the permanent magnet when same are adhered, and the cracking of the permanent magnet which occurs when the permanent magnet is secured to the shaft. Accordingly, a rotor construction utilizing a brittle high performance permanent magnet, and a method for forming same is desired.